varying sensitivities on the ABI310

Hi
Has anyone ever encountered this problem running Genescan on the 310?
We've
recently found a drop in sensitivity of detection such that we had to
triple the amt of Rox std to get a signal. The PCR products that we used
to
run at 1:25-1:50 dilution, we've had to run neat. The sizing has been very
reproducible across runs on different days but the sensitivity varies fr
day to day--one day we had to triple the amt of Rox, the next day we could
get by with double. It is q wasteful of Rox and we'd like to get to the
bottom of this!
Thanks
Agnes